Although speed reducing gear boxes employing planetary gear systems have long been known, they have generally suffered from gear chattering or ballotting on stopping or starting, owing to the inertial resistance of individual gears to changes in speed, as well as from unequal loading of their many gears, leading to shortened operating life. Ballotting is a particularly acute problem in stepping motor applications, in which frequent stops and starts are routine. It can cause premature gear failure due to vibration fatigue, as well as undesirably noisy operation.
Accuracy limitations of planetary gear boxes have also limited their application to stepping motors. Small individual gear inaccuracies, principally due to eccentricity, and less important in continuous rotation applications, can cause significant angular errors in the output of a stepping motor. The cumulative angular error resulting from the numerous gear meshes between input and output is a further limitation.